The goal of this project is to understand the molecular mechanism of sodium-phosphate cotransport and sodium-lithium countertransport. Na-PO4 cotransport is an important physiological process vital to all mammalian cells through its active transport of inorganic phosphate, a central metabolite. Na-Li countertransport is a medically important cellular mechanism that pumps lithium out of cells. Its rate in erythrocytes inversely correlates with the effectiveness of Li therapy in bipolar disease and directly correlates with the development of essential hypertension. Despite their importance, these processes are not well understood because the responsible molecules have not been identified or have not been adequately characterized in model systems. Recently there has been progress in both of these areas. There is circumstantial kinetic and pharmacological evidence that the "brain specific" gene, BNP1 (or a related isoform), is the Na-PO4 cotransporter in K562 erythroleukemic cells and erythrocytes. The project will test three hypotheses. Hypothesis I: BNP1 is the erythrocyte Na-PO4 cotransporter. This hypothesis will be tested in Specific Aim 1 by demonstrating that BNP1 is the major isoform present in human erythropoietic cells and K562 cells and in Specific Aim 2 by demonstrating that BNP1 protein is present in those cells and mature erythrocytes. Hypothesis II: the erythrocyte Na-phosphate cotransporter is also the major Na-Na exchanger. This hypothesis will be tested in Specific Aim 3 by determining the stoichiometry of Na-PO4 cotransport and in Specific Aim 4 by characterizing the new Na transport caused by the heterologous expression of BNP1 in oocytes and in HEK293 cells using a new inducible promoter system. Hypothesis III: the Na-PO4 cotransporter is the long sought molecular basis for Na-Li countertransport. This hypothesis will be tested in Specific Aim 5 by determining the new Na-Li countertransport in BNP1 expressing oocytes and HEK293 cells.